1. Field of the Invention
The invention relates to a process for bleaching alkyl polyglycosides, in which the alkyl groups have 8 to 24 C atoms, are bleached in aqueous solution by peroxy compounds in a tubular reactor.
2. Discussion of the Background
Alkyl polyglycosides are non-toxic and readily degradable surfactants. They are therefore used as washing and cleaning agents and as emulsifiers and dispersants. However, they only have the desired surfactant properties when the alkyl groups have at least 8 C atoms.
Alkyl polyglycosides having long-chain alkyl groups are generally prepared by single-stage or multistage syntheses.
A single-stage preparation process is described inter alia in DE-A-41 01 252.
A two-stage process is specified for example in EP-A-0 306 652, according to which an n-butyl glycoside is first prepared by glycosidation with n-butanol and the desired long-chain alkyl polyglycoside is thereupon prepared by transglycosidation with a long-chain alcohol.
During the preparation of the alkyl polyglycosides, product discolorations occur due to undesired oxidation and condensation processes. The technological properties are not affected by this. However, the disadvantageous visual impression makes it more difficult to market the products. In order to improve the color quality, alkyl polyglycosides are therefore generally subjected to a bleaching process after their preparation. For this purpose they are first neutralized and excess fatty alcohol is distilled off, whereupon the bleaching is then performed in aqueous solution.
According to EP-A-0 165 721, the color of alkyl polyglycoside solutions can be improved by multistage bleaching using hydrogen peroxide and stabilized by addition of compounds releasing sulphur dioxide. Because of the high volatility of hydrogen peroxide, this bleaching is carried out in a closed vessel at 1 to 20 bar.
Bleaching is normally performed in stirred tanks. However, these reactors have an unfavorable broad residence time spectrum for a continuous mode of operation and lead to a broad spectrum of reaction products. As a result, a high additional expenditure in terms of apparatus is required to avoid the disadvantages.
The object of the present invention was to provide an improved process for bleaching. In particular, the aim was to avoid the disadvantages of bleaching in the conventional equipment of the prior art.